Entities other than the utility power grid now frequently generate electrical power for local or dedicated uses, as well for connection to the utility grid to generate revenues and/or offset costs. Such systems may be referred to as having a dual, or multiple, sourced power bus.
In some electric power generating systems, the manner of managing the energy that will operate the electric generator may require auxiliary equipment, such as pumps and fans. An example is a power system which recovers heat, from such sources as geothermal wells, food processing plants or landfills, or the like, utilizing an Organic Rankin Cycle system.
For economic efficiency, it is desirable that a low cost generator provide power for all auxiliary equipment, while at the same time providing power which has shape (little or no harmonic distortion), power factor (PF), and frequency that are all suitable for interface with the utility power grid. Synchronous generators are typically more expensive and require additional controls compared with other, cheaper generators such as induction generators which, because of their construction, are less expensive than synchronous generators. However, induction generators have an inherently lower power factor (PF) than what is typically acceptable to utility grids. Moreover, to the extent certain types of non-linear loads, such as variable-speed drives with diode front ends, are associated with the auxiliary equipment, high levels of harmonic distortion may occur in the current.
An example of the conditions described in the foregoing paragraph may be seen in the characterization of the “prior art” (FIG. 1 therein) described in U.S. Pat. No. 7,038,329 (hereinafter '329) to S. J. Fredette, et al for “Quality Power From Induction Generator Feeding Variable Speed Motor”, the disclosure of which is incorporated herein by reference to the extent consistent and appropriate. Similar thereto is the following characterization herein of that “prior art” as it is depicted in FIG. 1 herein.
Referring to FIG. 1 herein of the “prior art”, there is depicted a single line of a typically multi-phase (typically 3-phase) power system in which an induction electric power generator 10, driven by some form of prime mover 12, is connected, or connectable, with the utility power grid 14 via power bus 16 including switch 18, typically a breaker or contactor. Moreover, there is depicted broadly in block form, one or more non-linear loads 20, typically including variable speed drives with diode rectifier front ends and associated with the auxiliary loads, operatively connected to the power bus 16. Those non-linear loads, and particularly the variable speed drives with diode rectifier front ends, are the same or similar to elements 11, 12, 13 and 16 of FIG. 1 of the aforementioned '329 patent. Because the induction generator requires reactive power to operate, the reactive power is provided by a power factor correction capacitor (Cpfc) 22 in order to maintain the power factor at the interface with the utility grid above limits imposed by accepted standards. The power factor limits are normally above 90%, and mainly above about 95%. The power factor correction capacitor(s) 22 is typically connected in shunt with the non-linear loads 20. Still further, to address the significant levels of harmonic distortion in the current that may be introduced by the non-linear loads 20, one or more harmonic filter(s) 24, is/are connected in series with the non-linear loads 20. A source inductance (Ls) 26 is represented in the power bus 16 as being the inductance of the power grid 14 and any interface transformer at or near the installation site, and an illustrated source resistance 27 is of similar origins.
As shown in FIG. 1 herein, as well as in FIG. 1 of the aforementioned '329 patent, the power factor correction capacitor (Cpfc) 22 and the harmonic filter(s) 24 are separate and distinct from one another. Stated another way, one may be said to be external to the other. Indeed, although the harmonic filter 24 may include a filter capacitor or capacitance in combination with one or more inductive impedance elements to provide the requisite filtering of the harmonics, such capacitor is separate from the power factor correction capacitor (Cpfc) 22.
The prior art configuration of FIG. 1 will be further understood in the context of prior art FIG. 2, which is substantially the same as FIG. 1 but configured to illustrate the harmonic filter 24 and the power factor correction capacitor 22 in greater detail and as separate from one another. The AC power grid and the induction power generator are collectively represented by section block 100 containing the AC source 114, the inductive source impedance 126, the source resistance 127, and induction generator 110. Correspondingly, the current to the various loads, including the non-linear loads, is represented by the current source symbol 120. Intermediate the AC source 114 and the output load current 120 are separate section blocks 122 and 124, representing the power factor correction capacitor 22 and the harmonic filter 24, respectively, of FIG. 1.
The power factor correction block 122 includes a power factor correction capacitor 22 having a detuning inductor (Ldet) 30 in series therewith, and is connected across the AC source 114 and across the induction generator 110. The inductor 30 is required to form a resonant tank so as to limit harmonic currents from flowing to the capacitor(s) and thereby causing excessive heating, which may be life-limiting.
Similarly, the harmonic filter section block 124 representing the harmonic filter 24 of FIG. 1 is also connected across the AC source 114 in a general “T” network including, more specifically, a “bridged-T”. The cross arm of the general T harmonic filter section block 124 includes several inductances arranged or depicted in series, including an input inductance (Lin) 32 shown connected at one end to the junction of the source inductance 126 or the source resistance 127 and the detuning inductance 30, and at the other end to the cross arm of a bridged-T comprising a parallel-bridged arrangement of a further inductance (L1) 34 connected in parallel with a series connection of a resistance (R) 36 and a filter inductance (Lf) 38. The cross arm of the general T filter network is completed by the connection at the junction of inductances 34 and 38 of one end of a still further inductance (L2) 40, the other end of which is connected to one side of the non-linear load(s) represented by the current source 120. A filter capacitor (Cf) 42 is connected at one end to the junction between the resistance 36 and the filter inductance 38 and is thereafter connected in shunt with the power source 114 and current source 120 to complete the “vertical arm” of the bridged-T filter.
While the afore-described arrangement is effective in maintaining acceptable power factor in the presence of the induction generator and of also reducing or eliminating the harmonic distortion introduced by the non-linear loads, especially as represented by the variable speed drives with diode rectifier front ends, it nonetheless comes at significant parts count and cost of equipment. More specifically, the ratings required of the filter capacitor(s) and the power factor correction capacitor(s) cause them to be relatively large and expensive. Although the insulated gate bipolar transistor switched bridge and associated control circuitry of the aforementioned '329 patent do provide a good quality of power without power factor correction and with little or no harmonic distortion, that circuitry itself comes at a significant cost or expense, such that it may not be a particularly acceptable trade-off.
Accordingly, what is needed is an arrangement in which an induction generator and associated non-linear loads are connectable to the utility power grid and operate with an acceptable power factor and minimal harmonic distortion, yet are reliable and cost effective in attaining that result.